perf: avoid unnecessary allocations in hot paths

[HazAT]

✅ Low risk — pure micro-optimizations, no new state or caching

Part of #2901 (reduce memory allocations by ~53%)

Changes

Backtrace::Line.parse:

• Use indexed regex captures (match[1]) instead of .to_a destructuring — avoids intermediate array allocation
• Add end_with? guard before .sub! for .class extension — skips string allocation on non-JRuby (99.9% of cases)
• Use match? instead of =~ in #in_app — avoids MatchData allocation
• Add nil guard for file in #in_app

StacktraceInterface::Frame:

• Remove stored @project_root/@strip_backtrace_load_path ivars (passed as args instead) — fewer entries in Interface#to_h
• Use byteslice instead of [] for filename prefix stripping
• Replace chomp(File::SEPARATOR) with end_with? check — avoids string allocation
• Inline under_project_root? to eliminate method call overhead

StacktraceBuilder#build:

• Replace select + reverse + map + compact chain with single reverse while loop — eliminates 3 intermediate array allocations

RequestInterface:

• Use delete_prefix instead of regex sub for HTTP_ prefix removal
• Replace key.upcase != key with key.match?(LOWERCASE_PATTERN) — avoids allocating upcased string for every Rack env entry
• Cache Rack version check — avoids repeated Gem::Version comparisons

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Cursor Bugbot has reviewed your changes and found 1 potential issue.

There are 4 total unresolved issues (including 3 from previous reviews).

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